Process for soft coating metals to be submitted to cold deformations



atented Jana Ml, 1933 UNETED STATES PATENT QFFECE FRANCESCO GIANNI, OF MILAN, ITALY, ASSIGNOR F ONE-HALF '10 ALESSANDRO SALVI, 0F MILAN, ITALY PROCESS FOR SOFT COATING METALS TO BE SUBMITTED TO GOLD DEFORLIATIONS No Drawing. Application filed July 14, 1931, Serial No. 550,818, and in Italy February 14, 1931.

It is well known that in order to obtain the best results in cold-drawing of iron and similar metals, the material is coated with a thin layer of soft metal, as for instance lead.

Cold-rolling or cold-drawing can be obtained at the ordinary temperature of the surrounding, without intermediary annealings, with a small consumption of power and a reduced number of steps passages.

Various processes have been proposed for obtaining a convenient lead-coating of the metals to be treated as above, which coating must suit the condition of being very thin and easily removable, but all processes proposed heretofore have shown the serious inconvenience that the removal of the lead, afterthe completion of the mechanical operations, is not always possible and in any way leaves a tarnished and rough surface, this depreciating the product and making it unacceptable for certain particular purposes. Most of said processes require also long heat-treatments which are neither easy nor economical.

The object of the present invention is to prepare a slight layer of soft plastifying metal as lead, to form a coating for the metal to be cold rolled or drawn or deformed in any other manner, such a coating being easily removed so as to leave a perfectly smooth and bright surface. 7

It is a known law that metals having a certain electrolytic pressure when immersed in a cold solution of a salt of a metal having a lower electrolytic pressure, are covered by a layer of this metal. Experience has shown however that iron or steel immersed in a cold solution of a metal salt may not be covered by a uniform, consistent and adherent layer, even though the salt of said solution is a salt of a metal (for instance, lead) having an electrolytic pressure lower than that of iron. In this case the metal of said salts leaves on the surface of the iron article an incoherent granulation instead of forming a proper continuous layer.

Furthermore it is well known that iron and its alloys do not present the property of amalgamatmg: Mercury brought into contact with an iron surface is not proper to amalgamate it but only it may adhere leaving a very thin veil.

According to the invention, said property may be utilized to obtain on iron surfaces a consistent layer of another metal (for instance, lead) which is not'per se proper to deposit electrolytically a continuous layer on iron, but which may be, on the other hand, easily amalgamated. As matter of consequence, the covering soft metal is not attached directly on'the covered surface but it remains separated therefrom through the veil of mercury which lies between the iron and said soft metal.

Such a consequence is very interesting and valuable, since the covering layer may be easily carried away at the completion of all mechanical workings, leaving the surfaces of the treated articles perfectly smooth and quasi specular.

This new coating process is characterized by the feature that the metal to be treated is brought into intimate contact with a metal forming the coating, said latter metal being in a state of disaggregation, or in a colloidal or nascent state, preferably dispersed in a neutral liquid or in a liquid capable of corroding slightly the surface of the metal'to be coated, the latter having undergone any process of slight previous superficial amalgamation.

A previous amalgamation ,of the metal to be coated had been already proposed before passing it through molten lead: but, ac-

cording to said process, mercury volatilizes as soon as the material is dipped in the bath and has no real effect. On the other hand the lead coating obtained either by passing the material in molten lead baths, or by treatment with solutions of lead-salts conveniently warmed, represents a slow process, and requires a previous pickling: it obtains also a coating which is difficult to remove and leaves a non-bright and granular surface when removed.

Ihe present invention can be achieved by means of a temporary superficial amalgamation of the metal to be treated, which amalgamation is obtained by any of the already known processes.

Subsequently said metal is placed, cold, in intimate contact with the disaggregated coating metal: said latter metal can be used finely powdered, either dry or moistened or colloidally distempered in any convenient neutral or active liquid: or else in the nascent state in solutions of salts, eventually acidified.

According to circumstances, intimate contact can be brought about by mechanical action, or can be self-obtained in virtue of physio-chemical interactions, but always without any heating whatever.

Very good results can be obtained for instance by treating the above metallic material for less than one minute in a diluted solution of bichloride of mercury (HgCl dipping it afterwards, cold in a 50% aqueous solution of an organic salt of the coating metal, acidified with a suitable organic acid.

The length of time required for this second treatment is always short, but must be graduated according to the thickness of the coating required for the intended mechanical treatment. This fact makes very economical the process according to the present invention As a matter of simple explanation, not hmiting in any way the invention to this partic ular process, it may be pointed out that a bath of 50% of acetate of lead acidified with about 5% of acetic acid and maintained at about 25-30 C. proves to be most convenient.

Once the mechanical treatments of the material are finished, its surface is covered with a very thin coating of amalgam on which is superposed a veil of the coating metal; by warming moderately, the mercury volatilizes, breaking the outside layer, so that a simple pickling operation enables to remove completely the metallic coating, leaving a surface perfectly smooth and bright, proper of being furtherly worked.

The process of amalgamation and the nature of the materials used for forming the protective coating may vary according to circumstances, without exceeding the limits of the present invention.

Vi hat I claim is:

1. A process for mechanically working pieces of iron to change their form, comprising covering with a thin film of mercury the surface of the starting pieces to be worked into a different form, chemically depositing on said film of mercury a thin continuous and tenacious layer of lead, mechanically working the coated starting pieces into its changed finished form, and then heating moderately said pieces so as to volatilize the mercury and by a pickle operation eliminate totally the external continuous layer of lead covering the finished pieces.

2. A process for mechanically working pieces of iron to change their form, comprising dipping the starting pieces, to be worked into a different form, in a water solution of bichloride of mercury, chemically depositing on said film of mercury a thin continuous and tenacious layer of lead, mechanically working the coated starting pieces into its changed finished form, and then heating moderately said pieces so as to volatilize the mercury and by a pickle operation eliminate totally the external continuous layer of lead covering the finished pieces.

3. A process for mechanically working pieces of iron to change their form, comprising covering with a very thin film of mercury the surface of the starting pieces to be worked into a different form, dipping said pieces in a bath of water solution of an organic salt of lead acidified with an organic acid, mechanically working the coated starting pieces into its changed finished form, and then heating moderately said pieces so as to volatilize the mercury and by a pickle operation eliminate totally the external continuous layer of lead covering the finished pieces.

4. A process for mechanically Working pieces of iron to change their form, comprising covering with a very thin film of mercury the surface of the starting pieces to be worked into a different form, dipping said pieces in a bath of water solution containing about 50% of acetate of lead, acidified with about 5% of acetic acid, and maintained at a temperature of about 2530 (1, mechanically working the coated starting pieces into its changed finished form, and then heating moderately said pieces so as to volatilize the mercury and by a pickle operation eliminate totally the external continuous layer of lead covering the finished pieces.

In testimony whereof I have hereunto set my hand.

FRANCESCO GIANN I. 

